SummaryAn improved distillation method is described for the quantitative determination of malonaldehyde in foods containing oxidized fats. The procedure is compared with other methods in current use for the determination of malonaldehyde. A high correlation of TBA numbers with rancid odor in cooked meats was established.

Acid-heat treatment is not necessary for the condensation of TBA with malonaldehyde nor for maximum colour development. Free malonaldehyde is produced during the oxidative breakdown of unsaturated fatty acids or food products. The amount of free malonaldehyde produced can be measured without acid-heat treatment.The reaction between malonaldehyde and TBA in water or 90% glacial acetic acid has been investigated at different temperatures. The results show that acid-heat treatment of the reaction mixture should be avoided, since the E%Oof the coloured complex is considerably affected by the acid. On the contrary, heating without acid accelerates the condensation of TBA with malonaldehyde without affecting the Ego.

Evidence is presented by UV, visible, and IR spectra as well as by paper and column chromatography of variously treated TBA‐acid reagents that the structure of TBA is altered upon acid‐heat treatment. A more pronounced but similar effect results from the treatment of the TBA with hydrogen peroxide. Some of the degradation products of TBA absorb at the same wavelength as the TBA‐malonaldehyde complex, as do many compounds which are reported in the literature to react with TBA. The significance of these findings in respect to the quantitative aspects of the test for the determination of rancidity in food products is discussed.

Spectral studies carried out indicate that the pigment of the cooked cured meats and that of the denatured nitric oxide haemoglobin is a. low-spin ferrous-porphyrin co-ordination complex. The spectra of the nitric oxide haemochrome prepared from haemin, derived from cured meat, and that of the acetone extract of the same cured meat give strong evidence that both co-ordination positions of the iron ion in these compounds are occupied by nitric oxide and that globin is not associated with the molecule. Further evidence is presented by electron paramagnetic resonance studies, which show that the acetone extract of the cured meat pigment is diamagnetic.The pigment of the fresh cured meats and that of the nitric oxide haemoglobin are identical. They are low-spin ferrous-porphyrin co-ordination complexes, but the one coordination position of the iron in these compounds is occupied by globin and the other by nitric oxide.A mechanism of the f,zding of the cured meat colour is presented, as well as theoretical considerations for the preparation of cured meats with stable colour.

The spectra of the cooked meat pigments have been re-investigated and interpreted in the light of quantum mechanical theories.The compound responsible for the colour of cooked meats has been identified as a highspin ferric-porphyrin co-ordination complex. The fifth and sixth co-ordination positions of the ferric ion of this compound are occupied by a carboxylate ion of the denatured globin molecule and by water, respectively. This compound is t o be referred t o as niettnyochrornogen. IntroductionThe brown or grey colour of cooked meats has been attributed to denatured globin haemichrome' and while there is no doubt that the iron ion of the metal porphyrin complex is in the ferric state,l very little is known on the ligands occupying the fifth and sixth co-ordination positions of the ferric ion or the spin state of the metal ion of this compound.The grey or brown pigment of cooked meats has been characterised as a mixed denatured globin-nicotinamide haemichrome, by reduction and comparison with known haemochrom(es)ogens. Therefore, the exact nature of the pigment before reduction takes place is still unknown. Furthermore, the spectrophotometric evidence presented by Tappe12 does not cover the entire range of the visible spectrum, but the region between 400 and 600 mp only. I t is the region of the visible between 600 and 700 m p that gives conclusive evidence on the spin state of the ferric p~r p h y r i n s .~ According to quanturti mechanical theories, the absorption or reflectance spectra of the metal co-ordination complexes can arise from the following electronic transitions initiated by absorption of light :4The haem pigments of cooked meats have not been fully identified as yet. (u) ,z-electrons ( b ) Non-bonding electrons of the ligand (c) Non-bonding electrons of the metal (d) Bonding electrons between metal and ligand.It is, therefore, obvious that the absorption or reflectance spectra of the metal co-ordination compounds are closely related with the electronic configuration of both the co-ordinated metal and the ligands which occupy the fifth and sixth co-ordination positions of the metal ion.Measurements of magnetic susceptibility have often been employed for the elucidation of the electronic configuration of many metal c o m p l e x e~,~ including the iron-porphyrins.6-8 Once the configuration became known, the interpretation of their absorption spectra and the association of the various -bands with transitions of electrons of both metal ion and ligands has been easily a~c o m p l i s h e d ,~~ +12 particularly after the development of the crystal field theory. $ In the case of the iron.-porphyrins, it is generally recognised that the iron ion can be either in the ferrous or ferric state. Likewise, the electronic configuration of the iron in each valence state can be either low spin (covalent) or high spin (ionic).1° There is a suggestion that a third form of spin 3 / 2 (three unpaired electrons) is found in a very limited number of cases, e.g., in p h t h a l~c y a n i n e s ,~~ or in alkaline haemog...